Systems and methods for measuring trace contaminants in gas matrix using integrated cavity output spectroscopy

ABSTRACT

A laser absorption spectrometry system for gas measurement is provided. The system includes an integrated cavity output spectroscopy (ICOS) assembly including a gas cell. The gas cell includes a cell body defining an optical cavity and an input mirror and an output mirror positioned in the optical cavity. The assembly further includes a tunable diode laser configured to emit laser light, a collimator positioned in an optical path between the tunable diode laser and the gas cell, and a detector arranged to measure laser light exiting the optical cavity. The collimator includes at least one of a reflective collimation mirror or a gradient index (GRN) lens.

BACKGROUND

The field of the disclosure relates generally to systems and methods ofmeasuring gases, and more particularly, to systems and methods ofmeasuring contaminants in gases using integrated cavity outputspectroscopy (ICOS).

Whenever fuel gas such as natural gas, coal syngas, or biogas, isgenerated, transferred, or used, levels of contaminants are typicallyrequired for the process. Measurements of various contaminants, e.g.,H₂S, H₂O, O₂, and CO₂, are critical in preventing infrastructure damagedue to corrosion or chemical reactivity. Natural gas producers mustclean extracted gas to remove contaminants and then verify residuallevels before introducing natural gas into a pipeline. Desulfurizer bedsin fuel reformers need periodic replacement or regeneration to preventH₂S breakthrough into the reformed fuel product, and therefore frequentcontaminant level monitoring is needed.

ICOS is a powerful tool in measuring gases. Known system and methods aredisadvantaged in some aspects and improvements are desired.

BRIEF DESCRIPTION

In one aspect, a laser absorption spectrometry system for gasmeasurement is provided. The system includes an integrated cavity outputspectroscopy (ICOS) assembly including a gas cell. The gas cell includesa cell body defining an optical cavity and an input mirror and an outputmirror positioned in the optical cavity. The assembly further includes atunable diode laser configured to emit laser light, a collimatorpositioned in an optical path between the tunable diode laser and thegas cell, and a detector arranged to measure laser light exiting theoptical cavity. The collimator includes at least one of a reflectivecollimation mirror or a gradient index (GRN) lens.

In another aspect, a laser absorption spectrometry system for gasmeasurement is provided. The system includes an off-axis ICOS assembly.The assembly includes a gas cell including a cell body defining anoptical cavity and an input mirror and an output mirror positioned inthe optical cavity. The assembly further includes a tunable diode laserconfigured to emit laser light coupled off-axis into the optical cavity,and a detector arranged to measure laser light exiting the opticalcavity. The system further includes an ICOS calibration computing deviceincluding at least one processor in communication with at least onememory device. The at least one processor is programmed to determine aparameter of the ICOS assembly based on at least one of data collectedby the detector or simulation data.

In one more aspect, a laser absorption spectrometry system for gasmeasurement is provided. The system includes an off-axis ICOS assemblyincluding a gas cell. The gas cell includes a cell body defining anoptical cavity and an input mirror and an output mirror positioned inthe optical cavity. The assembly further includes a tunable diode laserconfigured to emit laser light coupled off-axis into the optical cavity,and a collimator positioned in an optical path between the tunable diodelaser and the gas cell. One or more parameters of the off-axis ICOSassembly are optimized to reduce etalons caused by the laser light beingreflected back and forth between surfaces in the off-axis ICOS assembly.

DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings.

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a schematic diagram of an integrated cavity outputspectroscopy (ICOS) system.

FIG. 2A is a schematic diagram showing the operation of the system shownin FIG. 1 .

FIG. 2B shows representative absorption spectra of gases.

FIG. 3 shows effects of etalons on measurements of gases.

FIG. 4A is a simplified diagram of optical components of the systemshown in FIG. 1 .

FIG. 4B shows formation of etalons.

FIG. 4C is a table listing etalons.

FIG. 4D shows amplitude spectra with the input laser at a wavelength of1574.5 nm.

FIG. 5A shows light trajectories when a lens is used as a collimator.

FIG. 5B shows light trajectories when a reflective collimation mirror isused.

FIG. 6A shows an exemplary arrangement of an angled collimator.

FIG. 6B shows effects of launch angles of a laser on etalons.

FIG. 7A shows effects of a radius of curvature and cavity length onetalons.

FIG. 7B shows effects of a radius of curvature and cavity length onetalons in 3D plots.

FIG. 8A shows formation of mirror etalons.

FIG. 8B shows light trajectories.

FIG. 8C shows light trajectories when a radius of curvature of the inputmirror is reduced.

FIG. 8D shows effects on etalons from the radius of curvature of theinput mirror.

FIG. 9 shows effects on etalons from divergence angles of collimator.

FIG. 10A shows effects on etalons from launch distances of laser.

FIG. 10B shows relationship between etalon frequency optical distancesand launch distances.

FIG. 11 is a flow chart of an exemplary method.

FIG. 12 is a block diagram of an exemplary computing device.

FIG. 13 is a block diagram of an exemplary server computing device.

DETAILED DESCRIPTION

The disclosure includes systems and methods of measuring tracecontaminants in gas matrix using integrated cavity output spectroscopy(ICOS). Method aspects will be in part apparent and in part explicitlydiscussed in the following description.

Measurement of various contaminants, e.g. H₂S, H₂O, O₂, and CO₂, in fuelgas is needed for preventing infrastructure damage and for compliancewith operation requirements. Corrosion from H₂S, CO₂, H₂O and O₂ causeleaks to downstream assets. H₂S is deadly even at low parts per million(ppm) values. Excess H₂O leads to hydrates that decrease flow capacityand potential blockage. Excess O₂ degrades gas processing chemicals suchas amines. In addition, H₂S, CO₂, H₂O and O₂ have no energy value, andtherefore are desirable to be removed from fuel gases.

In known systems, separate analyzers are used to analyze amounts oftrace contaminants for individual contaminants. Optical interferencesfrom strong broadband absorbers such as CH₄ in the natural gas matrixtypically limit the performance of spectroscopic measurements. Fastresponse is often enabled by flowing large amounts of gas through theanalyzer and therefore exchanging gas quickly inside the analyzer. Oftenthe gas is not returned to the process and instead is released into theenvironment, which is costly and pollutes the environment because CH₄ isa green-house gas. As a result, using multiple analyzers increases thecost and pollution.

In contrast, systems and methods described herein use one ICOS system toanalyze multiple contaminants by injecting a plurality of lasers at aplurality of nominal wavelengths into a gas cell of the ICOS system,thereby reducing costs and pollution to the environment and reducingresponse time.

In systems and methods described herein, a collimation lens may bereplaced with a reflective collimation mirror, which facilitatesinjection of lasers at different wavelengths and reduces opticalinterferences or etalons, thereby simplifying the system design andincreasing the accuracy and precision of measurements of contaminants.The accuracy and precision may be further improved by adjustingparameters of the ICOS system, such as the radius of curvature of cavitymirrors of the ICOS system, the cavity length, the launch angle of thelaser beam, the divergence angle of the laser beam, and the launchdistance of the laser beam. The parameters may be adjusted jointly orseparately. Further, a gradient index (GRIN) lens may be used in thecollimator to reduce etalons.

FIG. 1 is a schematic diagram of an exemplary laser absorptionspectrometry system 100. In the exemplary embodiment, system 100includes an ICOS assembly 102 and a plurality of pumps 104. Pumps 104are mounted in parallel such that one pump is running while other pumpsare on standby. Running pump 104 is changed periodically to avoidoveruse of any one pump 104. Periodic validation of the pump health andperformance is conducted and allows predictive maintenance activities tobe scheduled. A fault on pump 104 may be detected during the periodicvalidation and predictive maintenance may be performed before failure. Afault on pump 104 may also be detected immediately by a pressure sensor,reducing or eliminating downtime by switching to a back-up pump 104instantaneously.

In the exemplary embodiment, ICOS assembly 102 includes one or morediode lasers 106, a gas cell 116, and a detector 112. Diode laser 106may be a tunable near-infrared diode laser, where diode laser 106 may betuned to emit laser light in a certain wavelength range. ICOS assembly102 further includes a collimator 108 for focusing the laser lightemitted by lasers 106. Collimator 108 may include a collimation lens. Acollimation lens may be a spherical or aspherical lens. In someembodiments, collimator 108 includes a reflective collimation mirror 501(see FIG. 5B described later). In other embodiments, collimator 108includes a GRIN lens. Gas cell 116 includes a cell body 120 defining anoptical cavity 122. Gas cell 116 further includes an input mirror 118-iand an output mirror 118-o. Input and output mirrors 118 are positionedat two opposing ends of optical cavity 122. Input and output mirrors118-i, 118-o are highly reflective, having reflectance approximately99.995%.

In the exemplary embodiment, system 100 may further include an ICOScomputing device (not shown) and a laser controller 110. Lasercontroller 110 and the ICOS computing device may be part of ICOSassembly 102, or may be separate components from ICOS assembly 102.Laser controller 110 is configured to control diode laser 106 of ICOSassembly 102. Signals detected by detector 112 of ICOS assembly 102 arereceived and processed by the ICOS computing device. System 100 mayinclude an ICOS calibration computing device 114 configured to calibrateand optimize parameters of system 100. ICOS calibration computing device114 may be included in the ICOS computing device, or may be a separatecomputing device from the ICOS computing device. ICOS calibrationcomputing device 114 may be in communication with detector 112, throughwired or wireless communication. In some embodiments, ICOS calibrationcomputing device 114 is a server computing device. In one example, ICOScalibration computing device 114 may receive data collected by detector112 through a portable storage device, such as a flash drive or a thumbdrive.

FIGS. 2A and 2B shows the operation mechanism of system 100. FIG. 2A isa schematic diagram showing the operation of system 100. FIG. 2B showsrepresentative absorption spectra of gases. In operation, laser light202 from diode lasers 106 (FIG. 1 ) is coupled off-axis or at a non-zeroangle with z-axis or axis 602 of optical cavity 122 (see FIG. 6Adescribed later) into gas cell 116 through input mirror 118-i and exitfrom output mirror 118-o while gas to be analyzed flows into opticalcavity 122 through a gas inlet 204 and exit from optical cavity 122 froma gas outlet 206 of cell body 120. An ICOS assembly operated with lightcoupled off-axis into gas cell 116 may be referred to as an off-axisICOS assembly. Light 202 is reflected back and forth within opticalcavity 122, travelling a relatively long effective optical path lengthand therefore increasing the interaction path length between light 202and gas 208. As a result, the optical absorption of gas is increased toa detectable level and is used to quantify gas concentrations, even forgases at low concentrations or weakly absorbing gas. Near-infrareddetector 112 measures the light exiting from cell body 120 and themeasurements from detector 112 are used to measure gases. As shown inFIG. 2B, different gases have different absorption spectra. The spectraare used to measure the content of gases in fuel gases.

Referring back to FIG. 1 , system 100 is configured to measure multiplecontaminants using one ICOS system, unlike in a known method, whereseparate analyzers are used to measure different contaminants, incurringadditional costs and having increased negative environmental impact andfuel gas wastage.

In the exemplary embodiments, system 100 includes a plurality of lasers106 having a plurality of nominal wavelengths. In one example, tomeasure contaminants H₂S, CO₂, H₂O, and O₂, three lasers 106-1, 106-2,106-3 are used. Laser 106-1 has a nominal wavelength of 1574.5 nm. Laser106-2 has a nominal wavelength of 1314 nm. Laser 106-3 has a nominalwavelength of 760 nm. During the operation of laser 106, a range ofwavelengths below and/or above the nominal wavelength of laser 106 isscanned across. For example, if a laser 106 having a nominal wavelengthof 1574.5 nm is used, a wavelength range of 1574.1 nm-1575.3 nm may bescanned across in operation, where the wavelength of output light bylaser 106 may range from 1574.1 nm to 1575.3 nm. The selected nominalwavelengths correspond to absorption ranges of target gases. At theabsorption ranges, the target gases are relatively absorbent and theabsorption yields signals detectable by detector 112. For example, thenominal wavelength of 1574.5 nm corresponds to absorption ranges of H₂Sand CO₂. Because H₂S and CO₂ have different absorption spectra,measurements of H₂S and CO₂ may be obtained by using one laser at onenominal wavelength and separating the measurements using the differentabsorption spectra. The nominal wavelength of 1314 nm corresponds to theabsorption range of H₂O. The nominal wavelength of 760 nm corresponds tothe absorption range of O₂. The laser light emitted by lasers 106-1,106-2, 106-3 are combined in a combiner 124 into single laser light. Thecombined laser light may be sequential where laser light at differentnominal wavelengths is emitted sequentially. Alternatively, laser lightfrom lasers 106 is multiplex using dichroic mirrors or fiber combiners124 that use wavelength division multiplexing. In some embodiments,combiner 124 is not used. The plurality of lasers are separatelylaunched or injected into optical cavity 122. The number of lasers 106that may be included in system 100 is limited by the size and mount ofcollimator 108. Therefore, a reduced-sized optical cavity 122, such asan optical cavity having 1 inch (2.54 cm) in diameter, may not havephysical space for two or more launching devices for a plurality oflasers. In some embodiments, the positions of collimator 108 are offsetfrom one another.

Referring back to FIG. 2A, combined laser light 202 is input into acollimator 108 and injected into gas cell 116. As a result, system 100is used to measure concentrations of all trace contaminants with onesingle gas cell 116, reducing gas volume and methane leakage andresulting in shortened response time to allow for real-time monitoringof contaminants. Real-time monitoring of H₂S, CO₂, H₂O and O₂ allowstriggering of threshold alarms to redirect contaminated streams thatwould otherwise compromise safety and operational yield.

Besides being configured to measure two or more contaminants using oneanalyzer, instead of multiple analyzers, system 100 also providesflexibility over conventional systems. System 100 may be used to measureone, two, or more contaminants, and may be used to measure anycombinations of contaminants. H₂S, CO₂, H₂O, and O₂ are described hereinas examples only. System 100 may be used to measure other contaminantsor any combination of H₂S, CO₂, H₂O, and O₂ in addition to othercontaminants. Lasers having wavelengths corresponding to absorptionranges of other contaminants may be used or added to system 100.Alternatively, lasers may be tuned to the wavelength rangescorresponding to the absorption range of the contaminants to bemeasured.

The absorption spectra of gases, however, are affected by etalons, whichare caused by overlapping laser lights. Etalons may have sinusoidalwaveforms. FIG. 3 is a plot of exemplary spectra showing the effects ofetalons on absorption spectra. When laser light travels back and forthbetween parallel or quasi-parallel surfaces, the laser light and itsreflection interfere with one another, creating sinewave in theabsorption spectra. As a result, an absorption peak 301 and a spectrum302 of the gas are deformed by the etalons into peaks 301-d and spectra302-d.

FIGS. 4A-4D show the formation and effects of etalons in furtherdetails. FIG. 4A is a simplified diagram of optical components of system100. FIG. 4B shows three etalons 402-1, 402-2, 402-3, which are majorcontribution of system noise. FIG. 4C is a table listing significantetalons. FIG. 4D shows the mean, variance, and amplitude/power spectraof etalons obtained by applying a Fourier transform to raw absorptionspectra. The input light is at a wavelength of 1574.5 nm. Opticalcomponents of system 100 include collimator 108, input mirror 118-i,output mirror 118-o, a collecting lens 404, and photodetector 112 (FIG.4A). Etalon 402-1 may occur when light 202 travels one length (L) 406 ofoptical cavity 122. Cavity length 406 is the distance between outer side403 of input mirror 118-i and outer side 403 of output mirror 118-o(FIG. 4B). Etalon 402-2 may occur after the light has travelled acrossoutput mirror 118-o and reentered back as re-entrant. Etalon 402-2typically occurs after light has travelled seven times of cavity length406. Etalon 402-3 may occur when light reflected back to collimator 108.FIG. 4C lists most significant etalons. FIG. 4D shows plot 408, 408-1depicting the mean, variances, and amplitude spectra of etalons. In plot408-1, the x-axis of the distance travelled is in a logarithmic scale.As shown, etalons 402-1, 402-2, 402-3 are disruptive in the measurementsof gas absorption spectra. Although system 100 involves hundreds orthousands of reflections within optical cavity 122 for a singlemeasurement, the first several reflections contribute disproportionallyto self-interference noise because the light beam has not divergedsignificantly at the early travel time. The most disruptive etalons arethose associated with optical distances between optical surfaces thatproduce interfering sine waves with a semi-period, or a half of theperiod, equivalent in dimension to be within the absorption bandwidth athalf-height of the gases of interest, e.g., 402-3. Etalon 402-3 is moredisruptive than other etalons 402-1 and 402-2 because etalon 402-3affects absorption peaks of gases of interest more than other etalons,reducing measurement accuracy and precision.

In known ICOS systems, etalons are mitigated using piezo-electricactuators placed on input and/or output mirrors to modulate the cavitydimensions at a frequency vastly different from the laser scanningfrequency. The known approach, however, is complicated and expensive.Further, because etalons move in phase and amplitude as temperaturevaries, known systems and methods would need to be designed, redesigned,or adjusted to take into consideration of the effects of temperature onetalons, increasing complexity and costs of known systems. In addition,the known approach is designed for a system input with a laser at asingle nominal wavelength.

Systems and methods described herein overcome the above describedproblems in known systems and methods. A plurality of lasers may be usedin system 100 such that multiple gases are analyzed by one system 100,instead of multiple ICOS systems. A reflective collimation mirror mayreplace a collimation lens to reduce etalons and increase theperformance with a plurality of lasers. Parameters of system 100, suchas a radius of curvature of input mirror 118-i, the cavity length, thedivergence angle of the injected laser beam, the launch angle of thelaser beam, and the launch distance of the laser beam, may be adjustedto select optimized parameters with etalons reduced or minimized. A GRINlens may be used in collimator 108 to reduce etalons. The parameters maybe adjusted separately or jointly in any combination. System 100 mayinclude one or more features described herein in any combination. Forexample, a collimation lens may be used and one or more parameters ofsystem 100 are adjusted.

In some embodiments, collimator 108 includes reflective collimationmirror 501 (FIG. 5B). Reflective collimation mirror 501 has a reflectivesurface 502. The reflectance of reflective surface 502 may be greaterthan 96%. Reflective surface 502 may be coated with metal. Reflectivesurface 502 is parabolic or being a segment of a paraboloid. In oneexample, the end surface of optic fiber 504 that faces reflectivecollimation mirror 501 is angled, where an end surface 510 of opticfiber 504 forms a non-90° angle with optic fiber 504 in the lengthdirection.

FIGS. 5A and 5B provide a comparison between a collimation lens 506 anda reflective collimation mirror 501. Because of parallel orquasi-parallel surfaces between laser optic fiber 504, collimation lens506, and input mirror 118-i, etalons caused by the parallel orquasi-parallel surfaces may include fiber to lens etalons, lens tomirror low reflectivity side etalons, lens to mirror high reflectivityside etalons, and mirror etalons. When a reflective collimation mirror501 replaces collimation lens 506, the number of parallel orquasi-parallel surfaces between laser optic fiber 504 and reflectivecollimation mirror 501 or between reflective collimation mirror 501 andinput mirror 118-i is significantly reduced. Parallel refection insidecollimation lens 506 does not occur with reflective collimation mirror501. Reflections from input mirror 118-i are reflected away or outsideof gas cell 116. Surfaces 508 of reflective collimation mirror 501,including reflective surface 502, and end surface 510 of optic fiber 504are angled in such a way that light does not return on the same path.All of the changes lead to reduction of etalons.

A reflective collimation mirror 501 is advantageous when a plurality oflasers at different wavelengths are used. Because reflective collimationmirror 501 reflects light, unlike in collimation lens 506, which alsorefracts light, chromatic aberration does not occur with reflectivecollimation mirror 501 due to effects of wavelength on refraction butsimilar reflectance over wavelengths due to metallic coating versusanti-reflection coating in collimation lens 506. Therefore, one singlereflective collimation mirror 501 may be used in system 100 havinglasers in a plurality of nominal wavelengths.

Having one single reflective collimation mirror 501 in system 100 havinglasers in a plurality of nominal wavelengths is advantageous foradditional reasons. Replacing collimation lens 506 with reflectivecollimation mirror 501 shortens the overall length of the spectrometerassembly, enabling a compact configuration. Further, a single laserlaunch assembly may be used for a plurality of lasers, instead of usingtwo or more laser launch assemblies, thereby simplifying theoptomechanical assembly. Using reflective collimation mirror 501 alsoreduces cost by reducing the number of parts needed in system 100 andreducing the time and labor in manufacturing, quality control, andservice because only one beam alignment is needed, instead of two ormore beam alignment being needed in a typical system. Moreover,reflective collimation mirror 501 reduces effects of wavelengths on thetravel paths of light from the plurality of lasers, thereby simplifyingthe measurements of a plurality of gases using one ICOS system.

In some embodiments, collimator 108 is a GRIN lens. A GRIN lens has agradient profile such that a refractive index of the lens varies in adirection perpendicular to the optical axis of the lens. In anembodiment, the refractive index varies according to the followingequation:

$\begin{matrix}{{N = {N_{0}\left\lbrack {1 - {\left( \frac{k}{2} \right)r^{2}}} \right\rbrack}},} & \left( {{Eq}.1} \right)\end{matrix}$

where N₀ is a base refractive index corresponding to the center of thelens, k is a gradient constant, and r is a radius variable thatrepresents a distance from the center of the lens. In one embodiment,the GRIN lens is cylindrical, with a diameter in a range between 0.5 mmand 3 mm, which is smaller than a diameter of conventional spherical oraspheric lenses (e.g., plano-convex or bi-convex) that may start at 5 mmor more in diameter. For example, it may be difficult to grind, polish,or mold from a polymer material a spherical or aspheric lens at smallsizes with an appropriate focal length. Further, conventional sphericalor aspheric lens below, e.g., 5 mm in diameter, may not be availablecommercially at a low cost or produced without custom equipment. Incontrast, a GRIN lens that is commonly commercially available at a lowcost may be approximately 1.0 mm in diameter.

Use of a GRIN lens instead of a typical spherical or asphericalrefractive lens reduces etalons. A GRIN lens has a smaller size than atypical spherical or aspherical refractive lens, thereby reducing theinitial beam size entering into optical cavity 122 and in turn reducingetalons.

In the exemplary embodiment, the launch angle of light into gas cell 116may be adjusted. FIG. 6A show an exemplary arrangement of system 100.Collimator 108 is at an angle with input mirror 118-i. Although areflective collimation mirror 501 is depicted in FIG. 6A, adjusting thelaunch angle as described herein may be applied to system 100 having acollimation lens 506 or GRIN lens. FIG. 6B shows effects of a launchangle on etalons. A launch angle or an injection angle is the angle ofthe incident or impinging laser into optical cavity 122. Light intooptical cavity 122 may be assumed to have a Gaussian profile. A launchangle is calculated as the maximum laser intensity direction relative toz-axis 602 of optical cavity 122. A z-axis of an optical cavity is theoptical axis of cavity mirrors 118 or the centerline of optical cavity122. A launch angle may be represented as a pair of angles θx and θy,which are angles at which the light is tilted toward x-axis or y-axis,respectively. x-axis and y-axis are axes orthogonal to one another andto z-axis 602 of optical cavity 122 (see FIG. 6A). FIG. 6B is based ondata of system 100 having a reflective collimation mirror 501 andacquired with zero gas, which includes dry nitrogen (N₂). Zero gas doesnot absorb light and therefore measurements from detector 112 are systemnoise, such as etalons. Plots 604-1-604-9 are residual spectra at ninedifferent launch angles 606-1-606-9. Residual spectra are calculated asthe algebraic differences or point by point differences between a rawgas absorption (GA) spectrum, calculated as GA=(I/Io)−1, and a fitted orideal GA scale spectrum of the gas in gas cell 116. In plots604-1-604-9, the x-axis is wavelength and y-axis is the amplitude of theresidual spectra. Plot 604-c shows signal quality, including but limitedto signal to noise ratio (SNR), of system 100 as a function of launchangle 606. The signal quality is color coded, with black indicatingpoorest signal quality while white indicating highest signal quality.The x- and y-axes of plot 604-c are θx and θy angles in degrees, withcenter being zero, where the light is on-axis alignment with z-axis 602(FIG. 6A).

As shown in FIG. 6B, negative effects of etalons increase when amplitude608 of etalons increases, which are the y-axis values in plots604-1-604-9. Negative effects of etalons increase when the signalquality reduces (the dark regions in plot 604-c). Negative effects ofetalons also increase when the semi-period of the etalons is within therange half height bandwidth of the gas absorption bandwidth orapproximate to the half height bandwidth range such as within the rangeof third or quarter height bandwidth of the gas absorption bandwidth(plot 604-7). Based on the amplitude and period/frequency of etalons,among spectra 5, 8, and 9, spectrum 5 (plot 604-5) is marked as bestwhile spectra 8 and 9 (plot 604-8, 604-9) are marked as poor or worst,respectively. Angle 606-5 has a θx angle of 1.0° and θy of 0. Launchangle 606-9 corresponds to angle zero, where launch angle 606 is onaxis. Although in plot 604-7 the signal quality is relatively good andthe amplitudes of etalons are relatively low, the setup with angle 606-7(plot 604-7) produces disruptive exterior cavity elations that have asemi-period within or approximate the half-height bandwidth ofabsorption spectra (see etalons 402-3 in FIG. 4B), and should beavoided.

In operation, launch angle 606 may be selected based on the residualspectra when the sample gas is dry nitrogen. Launch angle 606 is variedand residual spectra are collected for each launch angle 606. Anoptimized launch angle is selected as a launch angle corresponding toresidual spectra having the lowest amplitude and/or semi-period of theetalons not being within a predetermined threshold such as a half-heightbandwidth, a third- or quarter-height bandwidth of the gas absorptionbandwidth, or any other range as determined by the measurement precisionrequirements.

When system 100 has a plurality of lasers at different nominalwavelengths, a reflective collimation mirror 501 is advantageous over acollimation lens 506. Because reflection is usually equal for differentwavelengths, a single launch angle may be optimized for differentwavelengths in system 100 that includes reflective collimation mirror501.

In the exemplary embodiment, radius of curvature of cavity mirror 118and cavity length 406 may be jointly adjusted to reduce etalons. As usedherein, a radius of curvature of cavity mirror 118 is the radius ofcurvature of inner side 401 of cavity mirror 118 (see FIGS. 8B and 8Cdescribed later). FIGS. 7A and 7B show beam overlaps inside opticalcavity 122 as a function of radius of curvature of cavity mirror 118 andcavity length 406 when launch angles θx and θy are 100 milliradian(mrad), based on computer simulation data. Beam overlaps cause etalons.FIG. 7A shows beam overlaps as a function of cavity length 406 for aradius of curvature of cavity mirror 118 at 250 mm, 500 mm, or 1000 mm,respectively. The y-axis is in an arbitrary unit. FIG. 7B shows threedimensional (3D) plots (plot 704 and a false color map 708) of theresults. Beam overlap peaks 702 are shown as dots in false color map708. In operation, a pair of radius of curvature and cavity length maybe selected as a pair that does not correspond to a beam overlap peak702. A look-up table may be generated by listing beam overlap peaks andcorresponding pairs of a radius of curvature and cavity length based onthe 3D plot 704 or false color map 708. A radius of curvature and/orcavity length may be selected by avoiding pairs listed in the look-uptable.

FIGS. 8A-8D show effects of a radius of curvature of cavity mirror 118alone on etalons. A reduced radius of curvature has reduced etalons.FIGS. 8A and 8B shows etalons occur when light is reflected back andforth between two sides of input mirror 118-i. When a radius ofcurvature reduces, inner side 401 of input mirror 118-i is less parallelwith outer side 403 of input mirror 118-i (FIG. 8C). As a result,etalons are reduced. FIG. 8D shows a comparison of etalons when theradius of curvature is 100 cm (plot 805-1) versus when the radius ofcurvature is 25 cm (plot 805-s). The x-axis of plots 805-1, 805-s arethe travel time or optical distance travelled. The y-axis is thedistance between the collimator 108 and input mirror 118-i. Spectra werecollected continuously while the distance between collimator 108 andinput mirror 118-i was slowly increased. The amplitude spectrum obtainedby applying a Fourier transform to the residual spectra of zero gas isdepicted as color maps in FIG. 8D. As shown in FIG. 8D, when using aninput mirror having a 100 cm radius of curvature, etalons are observedas marked by a dotted line 807 in an area 809. In contrast, when usingan input mirror having a 25 cm radius of curvature, etalons are notobserved at the same area 809. The radius of curvature of output mirror118-o may be the same as the radius of curvature of input mirror 118-iand may be adjusted at the same time.

FIG. 9 shows effects of divergence angles of injected laser beam onetalons. A divergence angle of a laser beam is the angle that the laserlight beam diverges. The divergence angle of a laser beam is affected bythe divergence angle of collimator 108, which is the angle of lightdiverging from the collimator. The divergence angle of a laser beam maybe changed by changing collimator 108 with a different divergence angle.FIG. 9 shows plots 902-s, 902-1 of etalons as a function time or opticaldistance traveled in the x-axis and a distance between collimator 108and input mirror 118-i in the y-axis. The amplitude spectrum obtained byapplying a Fourier transform to the residual spectra of zero gas isdepicted as color maps in FIG. 9 . The color maps indicate the level ofetalons. As shown, when the divergence angle of collimator 108 is 0.83°,etalons are observed as marked by dotted line 807 in area 809 (plot902-s). In contrast, when the divergence angle of collimator 108 is1.68°, etalons are not observed in the same area 809 (plot 902-1).Accordingly, an increase of divergence angle of collimator 108 reducesetalons.

FIGS. 10A and 10B show effects of launch distances on etalons. As usedherein, a launch distance is the distance between the collimator 108 andthe inner side 401 of the inner mirror 118-i (distance 405 in FIG. 4A).FIG. 10A shows amplitude spectra obtained by applying a Fouriertransform to the residual spectra of etalons measured using ambient air(plot 1002-a), scuba tank air (plot 1002-s), and zero gas (plot 1002-z).FIG. 10B is a plot of etalon frequencies as a function of launchdistance. The data was acquired using cavity mirrors having a 100 cmradius of curvature and a divergence angle of 0.83° for the injectedbeam. As shown in FIG. 10B, etalons are observed as marked by dottedline 807 in area 809. Amplitudes of etalons are negligible when thelaunch distance is between 55 mm and 65 mm.

In operation, to select an optimized range of launch distance, residualspectra are collected while varying the launch distance. The residualspectra or etalon amplitude may be plotted as a function of the launchdistance. An optimized launch distance or an optimized range of launchdistances may be selected based on the plot. For example, the optimizedlaunch distance or optimized range of launch distance is selected as thelaunch distance corresponding to the amplitude of etalons being lessthan a predetermined threshold such as the minimum amplitude, within 10%above the minimum amplitude, within 15% above the minimum amplitude, orany threshold as determined by the system requirements or specification.

The values of launch angles, launch distances, divergence angles, radiiof curvature, or cavity lengths provided above are examples only andspecific to a particular configuration of an ICOS assembly. The valuesmay change with optomechanical configurations of the ICOS assembly.

FIG. 11 is an exemplary method 1100 of calibrating parameters of an ICOSassembly. The ICOS assembly may be the ICOS assemblies described above.Method 1100 may be implemented with calibration computing device 114.Method 1100 includes receiving 1104 data of laser light. The data may besimulated data or data collected by detector 112. Data may be simulatedor collected while varying parameters of ICOS assembly. One or moreparameters may be varied at one time. Method 1100 further includesanalyzing 1106 the data. In addition, method 1100 includes determining1108 an optimized parameter or an optimized range of the parameter ofthe ICOS assembly. The parameter(s) may be optimized by minimizing orreducing noise to be below a predetermined threshold, or selecting theparameter(s) among the tested parameter(s) corresponding to the smallestlevel of noise. Noise may be noise from etalons.

Calibration computing device 114 described herein may be any suitablecomputing device 800 and software implemented therein. FIG. 12 is ablock diagram of an exemplary computing device 800. In the exemplaryembodiment, computing device 800 includes a user interface 804 thatreceives at least one input from a user. User interface 804 may includea keyboard 806 that enables the user to input pertinent information.User interface 804 may also include, for example, a pointing device, amouse, a stylus, a touch sensitive panel (e.g., a touch pad and a touchscreen), a gyroscope, an accelerometer, a position detector, and/or anaudio input interface (e.g., including a microphone).

Moreover, in the exemplary embodiment, computing device 800 includes apresentation interface 817 that presents information, such as inputevents and/or validation results, to the user. Presentation interface817 may also include a display adapter 808 that is coupled to at leastone display device 810. More specifically, in the exemplary embodiment,display device 810 may be a visual display device, such as a cathode raytube (CRT), a liquid crystal display (LCD), a light-emitting diode (LED)display, and/or an “electronic ink” display. Alternatively, presentationinterface 817 may include an audio output device (e.g., an audio adapterand/or a speaker) and/or a printer.

Computing device 800 also includes a processor 814 and a memory device818. Processor 814 is coupled to user interface 804, presentationinterface 817, and memory device 818 via a system bus 820. In theexemplary embodiment, processor 814 communicates with the user, such asby prompting the user via presentation interface 817 and/or by receivinguser inputs via user interface 804. The term “processor” refersgenerally to any programmable system including systems andmicrocontrollers, reduced instruction set computers (RISC), complexinstruction set computers (CISC), application specific integratedcircuits (ASIC), programmable logic circuits (PLC), and any othercircuit or processor capable of executing the functions describedherein. The above examples are exemplary only, and thus are not intendedto limit in any way the definition and/or meaning of the term“processor.”

In the exemplary embodiment, memory device 818 includes one or moredevices that enable information, such as executable instructions and/orother data, to be stored and retrieved. Moreover, memory device 818includes one or more computer readable media, such as, withoutlimitation, dynamic random access memory (DRAM), static random accessmemory (SRAM), a solid state disk, and/or a hard disk. In the exemplaryembodiment, memory device 818 stores, without limitation, applicationsource code, application object code, configuration data, additionalinput events, application states, assertion statements, validationresults, and/or any other type of data. Computing device 800, in theexemplary embodiment, may also include a communication interface 830that is coupled to processor 814 via system bus 820. Moreover,communication interface 830 is communicatively coupled to dataacquisition devices.

In the exemplary embodiment, processor 814 may be programmed by encodingan operation using one or more executable instructions and providing theexecutable instructions in memory device 818. In the exemplaryembodiment, processor 814 is programmed to select a plurality ofmeasurements that are received from data acquisition devices.

In operation, a computer executes computer-executable instructionsembodied in one or more computer-executable components stored on one ormore computer-readable media to implement aspects of the inventiondescribed and/or illustrated herein. The order of execution orperformance of the operations in embodiments of the inventionillustrated and described herein is not essential, unless otherwisespecified. That is, the operations may be performed in any order, unlessotherwise specified, and embodiments of the invention may includeadditional or fewer operations than those disclosed herein. For example,it is contemplated that executing or performing a particular operationbefore, contemporaneously with, or after another operation is within thescope of aspects of the invention.

FIG. 13 illustrates an exemplary configuration of a server computerdevice 1001 such as calibration computing device 114. Server computerdevice 1001 also includes a processor 1005 for executing instructions.Instructions may be stored in a memory area 1030, for example. Processor1005 may include one or more processing units (e.g., in a multi-coreconfiguration).

Processor 1005 is operatively coupled to a communication interface 1015such that server computer device 1001 is capable of communicating with aremote device or another server computer device 1001. For example,communication interface 1015 may receive data from calibration computingdevice 114, via the Internet.

Processor 1005 may also be operatively coupled to a storage device 1034.Storage device 1034 is any computer-operated hardware suitable forstoring and/or retrieving data, such as, but not limited to, wavelengthchanges, temperatures, and strain. In some embodiments, storage device1034 is integrated in server computer device 1001. For example, servercomputer device 1001 may include one or more hard disk drives as storagedevice 1034. In other embodiments, storage device 1034 is external toserver computer device 1001 and may be accessed by a plurality of servercomputer devices 1001. For example, storage device 1034 may includemultiple storage units such as hard disks and/or solid state disks in aredundant array of inexpensive disks (RAID) configuration. storagedevice 1034 may include a storage area network (SAN) and/or a networkattached storage (NAS) system.

In some embodiments, processor 1005 is operatively coupled to storagedevice 1034 via a storage interface 1020. Storage interface 1020 is anycomponent capable of providing processor 1005 with access to storagedevice 1034. Storage interface 1020 may include, for example, anAdvanced Technology Attachment (ATA) adapter, a Serial ATA (SATA)adapter, a Small Computer System Interface (SCSI) adapter, a RAIDcontroller, a SAN adapter, a network adapter, and/or any componentproviding processor 1005 with access to storage device 1034.

At least one technical effect of the systems and methods describedherein includes (a) injecting a plurality of laser light at a pluralityof wavelength ranges; (b) a reflective collimation mirror; (c) aplurality of pumps; (d) adjustments of system components to reduceetalons; and (e) a GRIN lens.

Exemplary embodiments of systems and methods of measuring contaminantsare described above in detail. The systems and methods are not limitedto the specific embodiments described herein but, rather, components ofthe systems and/or operations of the methods may be utilizedindependently and separately from other components and/or operationsdescribed herein. Further, the described components and/or operationsmay also be defined in, or used in combination with, other systems,methods, and/or devices, and are not limited to practice with only thesystems described herein.

Although specific features of various embodiments of the invention maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the invention, any feature ofa drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A laser absorption spectrometry system for gasmeasurement, comprising: an integrated cavity output spectroscopy (ICOS)assembly comprising: a gas cell comprising: a cell body defining anoptical cavity; and an input mirror and an output mirror positioned inthe optical cavity; a tunable diode laser configured to emit laserlight; a collimator positioned in an optical path between the tunablediode laser and the gas cell, wherein the collimator includes at leastone of a reflective collimation mirror or a gradient index (GRN) lens;and a detector arranged to measure laser light exiting the opticalcavity.
 2. The system of claim 1, wherein the system further comprisesan ICOS calibration computing device comprising at least one processorin communication with at least one memory device, and the at least oneprocessor programmed to: simulate the laser light traveling through theICOS assembly as a function of a radius of curvature of the input mirrorand a cavity length of the optical cavity; identify beam overlap peaksbased on simulated data of the laser light; and determine an optimizedradius of curvature and an optimized cavity length that do notcorrespond to one of the beam overlap peaks.
 3. The system of claim 1,wherein the collimator includes the reflective collimation mirror, thereflective collimation mirror having a parabolic reflection surface. 4.The system of claim 1, wherein the system further comprises an ICOScalibration computing device comprising at least one processor incommunication with at least one memory device, and the at least oneprocessor programmed to: receive data collected by the detector whenzero gas is input into the optical cavity, wherein the data arecollected a plurality of times, and during each collection, a parameterof the ICOS assembly is changed; determine residual spectracorresponding to the changed parameter based on the received data; anddetermine an optimized parameter based on the residual spectra.
 5. Thesystem of claim 4, wherein the parameter is a launch angle of the laserlight.
 6. The system of claim 4, wherein the parameter is a radius ofcurvature of the input mirror.
 7. The system of claim 4, wherein theparameter is a divergence angle of the laser light.
 8. The system ofclaim 4, wherein the parameter is a launch distance of the laser light.9. The system of claim 1, wherein the collimator includes the GRIN lens.10. A laser absorption spectrometry system for gas measurement,comprising: an off-axis integrated cavity output spectroscopy (ICOS)assembly comprising: a gas cell comprising: a cell body defining anoptical cavity; and an input mirror and an output mirror positioned inthe optical cavity; a tunable diode laser configured to emit laser lightcoupled off-axis into the optical cavity; and a detector arranged tomeasure laser light exiting the optical cavity; and an ICOS calibrationcomputing device comprising at least one processor in communication withat least one memory device, and the at least one processor programmedto: determine a parameter of the ICOS assembly based on at least one ofdata collected by the detector or simulation data.
 11. The system ofclaim 10, wherein the at least one processor is further programmed to:simulate the laser light traveling through the ICOS assembly as afunction of a radius of curvature of the input mirror and a cavitylength of the optical cavity; identify beam overlap peaks based on thesimulated data; and determine an optimized radius of curvature and anoptimized cavity length that do not correspond to one of the beamoverlap peaks.
 12. The system of claim 10, wherein the at least oneprocessor is further programmed to: receive the data collected by thedetector when zero gas is input into the optical cavity, wherein thedata are collected a plurality of times, and during each collection, aparameter of the ICOS assembly is changed; determine residual spectracorresponding to the changed parameter; and determine an optimizedparameter based on the residual spectra.
 13. The system of claim 12,wherein the parameter is a launch angle of the laser light.
 14. Thesystem of claim 12, wherein the parameter is a radius of curvature ofthe input mirror.
 15. The system of claim 12, wherein the parameter is adivergence angle of the laser light.
 16. The system of claim 12, whereinthe parameter is a launch distance of the laser light.
 17. The system ofclaim 12, wherein the at least one processor is further programmed to:determine the optimized parameter based on at least one of amplitudes,signal quality, or semi-periods of the residual spectra.
 18. A laserabsorption spectrometry system for gas measurement, comprising: anoff-axis integrated cavity output spectroscopy (ICOS) assemblycomprising: a gas cell comprising: a cell body defining an opticalcavity; and an input mirror and an output mirror positioned in theoptical cavity; a tunable diode laser configured to emit laser lightcoupled off-axis into the optical cavity; and a collimator positioned inan optical path between the tunable diode laser and the gas cell,wherein one or more parameters of the off-axis ICOS assembly areoptimized to reduce etalons caused by the laser light being reflectedback and forth between surfaces in the off-axis ICOS assembly.
 19. Thesystem of claim 18, wherein the one or more parameters are optimized toreduce the etalons caused by the laser light being reflected back andforth between the collimator and the input mirror.
 20. The system ofclaim 19, wherein the one or more parameters include at least one of alaunch angle of the laser light, a radius of curvature of the inputmirror, a divergence angle of the laser light, or a launch distance ofthe laser light.